(A) Superposition of urea-based inhibitors in the active-site of rhGCPII. The rhGCPII–inhibitor complexes were superimposed on corresponding Cα-atoms. The inhibitors are shown in stick representation and protein residues are shown as lines. Note invariant positioning of the P1′ glutamate contrasting with inhibitor conformational variability in the S1 pocket. (B) Hydrogen-bonding network in the S1 site of GCPII. Hydrogen bonding interactions (indicated with dashed lines) and distances (in Ångstroms) between 1 bound in the active site and S1 residues of GCPII. The zinc ions, chloride anion, and water molecules located in the active site are shown as blue, yellow and red spheres, respectively. The protein and inhibitor atoms are colored red (oxygen), blue (nitrogen), yellow (sulfur), violet (iodine), cyan (fluorine), gray (carbons).